Objectives Identify the inhibitory hypothalamic mechanisms mediating the social suppression of ovulation in subordinate female marmosets, using push-pull perfusion of conscious animals. In marmoset and tamarin monkeys, social status strongly regulates female fertility subordinate females typically undergo reproductive failure due to anovulation. Previous studies have indicated that this socially induced infertility is caused by suppressed pituitary release of gonadotropins, which in turn appears to be mediated by altered hypothalamic secretion of gonadotropin-releasing hormone (GnRH). In this study, we are characterizing the hypothalamic release of GnRH and of associated neurotransmitters in order to identify the hypothalamic mechanisms mediating this socially induced anovulation. Subjects are anovulatory subordinate female marmosets and dominant or pair-housed females undergoing ovulatory cycles. Each animal is initially implanted with a cranial pedestal. At least 6 weeks later, a miniaturized micromanipulator is used to lower a push-pull cannula into the stalk-median eminence. The monkey is then placed in a specially designed marmoset restraint for three days. On the third day, the animal undergoes push-pull perfusion of the stalk-median eminence for up to 8 h, with perfusate fractions of 230 l collected every 10 min. Cycling females are perfused during the mid-follicular phase of the ovarian cycle. Samples are assayed for GnRH and for a-endorphin or norepinephrine, neurotransmitters which are likely to inhibit GnRH release. Following perfusion, the micromanipulator and cannula are removed and the animal is returned to its home cage. Initial results indicate that in both cycling (N=6) and anovulatory (N=7) female marmosets, GnRH is released in a pulsatile manner. GnRH concentrations, pulse frequency, and pulse amplitude are comparable in dominant and subordinate females, but inter-pulse interval tends to be longer and more variable in subordinates. These results indicate that GnRH release is not suppressed in anovulatory subordinate female marmosets. Instead, subtle alterations in the pulsatile release of GnRH in these animals appear to be sufficient to inhibit gonadotropin secretion and ovulation. Key Words dominance, infertility, hypothalamus, gonadotropin-releasing hormone, push-pull perfusion